The field of the present invention is shaft sealing mechanisms for high-speed rotating machinery.
In high speed turbomachinery pressurized working fluid is introduced to a rotor within a turbine housing. The rotor is mounted to a shaft which is rotatably mounted within one or more bearings normally arranged outwardly of the rotor cavity of the turbine housing. A shaft seal is conventionally provided to avoid flow of the working fluid into the bearings. In high speed equipment a positive mechanical seal is often impractical. Consequently, labyrinth seals are frequently employed to seal the rotor cavity about the turbine shaft. Labyrinth seals employed in such applications tend to allow some flow of working fluid across the seal. This flow increases with a decrease in shaft speed such as experienced during shutdown.
In high speed turbomachinery employing such labyrinth seals, the adjacent bearing frequently is employed to provide the hermetic seal for the small amount of fluid leaking through the labyrinth seal. The working bearing about the turbine shaft can provide such a complete seal when lubricant is continuously provided to the bearing under positive pressure. Lubricant flowing from the bearing along the shaft toward the labyrinth seal often mixes with the working fluid passing through the seal. The fluids can be drawn off and separated for reuse.
In situations where the flow of pressurized lubricant to the bearing most adjacent the labyrinth seal in such turbomachinery is cut off, the pressurized working fluid passing through the labyrinth seal can drive the lubricant from the bearing. Under such circumstances, the hermetic sealing of the system at the bearing is lost. Furthermore, the working fluid is typically not a good lubricant and damage to the bearing can result from the loss of lubricant. Another adverse condition can also arise when there is low working gas pressure. In the area between the labyrinth seal and the bearing, reduced pressure might exist which would permit air or other fluid to leak through the bearing into this area and into the labyrinth seal under conditions when lubricant is not provided under pressure to the bearing.